The Science Behind Hunger

By Sarah MooreReviewed by Georgios Christofidis, Ph.D.

Obtaining food is a basic human need, like sleep, water, and adequate shelter. Feelings of hunger are an evolutionary tool to remind us to put fuel in our bodies to survive.

Image Credit: addkm/Shutterstock.com

This is known as "homeostatic hunger”. However, this is not the only type of hunger modern humans now experience, cultural cues, personal tastes, and reward systems also come to play in what is known as “hedonic hunger”.

In this case, the brain can make us experience hunger even when our bodies do not require calories.

Below we discuss the mechanisms underlying these different types of hunger.

The biological basis of homeostatic hunger

To ensure we remain healthy and consume enough nutritious and calorific foods the brain has evolved a complex circuitry to regulate blood and body levels of nutrients and to work to motivate us to consume more when we require more fuel.

This motivation comes from hunger, the body’s signal to us that we need more nutrition. Hunger causes us discomfort, which is alleviated when we eat, therefore, it serves the purpose of encouraging us to eat when the brain deems it necessary.

Much research has been conducted over the years to uncover the mechanisms underlying homeostatic hunger. Currently, scientists have a good idea of the components of these mechanisms, however, in some cases, their exact functioning is not clear. Below we discuss two of the most prominent regulatory systems involved in homeostatic hunger.

For a long time, scientists have understood hunger in the form of an antagonistic system, with the lateral hypothalamus causing feelings of hunger to encourage feeding, and the ventromedial nucleus having an inhibitory effect via feelings of satiation.

In the 1990s, scientists discovered that the hormone leptin plays a vital role in feelings of satiation. Since then, many studies have supported the hypothesis that feeding has the impact of increasing leptin levels in the blood, while food deprivation leads to a fall in leptin levels.

Research has found that humans and animals who lack either the leptin receptor or ligand have increased appetites and increased eating behaviors.

Leptin impacts the central nervous system regulation of feeding because the leptin receptor is a type 1 cytokine receptor that can activate the janus-kinase/STAT-3 pathway.

The long form of the leptin receptor can be found in numerous locations in the brain, although it is most abundant in the ventral basal hypothalamus, particularly within the arcuate, ventromedial, dorsomedial, and ventral premammalliary nuclei located within the hypothalamus.

Leptin binds to the receptors in these regions, triggering activity that leads to feelings of satiation. However, the exact physiological significance of leptin activity is not yet fully understood.

Ghrelin is another hormone that is regulated by eating and is believed to work in harmony with leptin. Studies have shown that ghrelin acts on the hypothalamus, particularly in the ventromedial and arcuate nuclei where an abundance of ghrelin receptors have been found.

Although, these receptors have also been found in other brain regions and the exact mechanisms of ghrelin action in the brain are still unclear.

However, the role of ghrelin has been highlighted by several studies. Notably, back in the early 2000s, a team of researchers measured ghrelin levels in those who had lost weight from dieting and those who lost it via a gastric bypass operation. They found that those who dieted had significantly increased levels of ghrelin, with peaks in the hormone before eating.

Those who had undergone surgery, on the other hand, showed reduced ghrelin levels and lacked the pre-meal peak. They were also more successful at maintaining their lower weights. This shows that ghrelin plays an essential role in hunger.

Homeostatic hunger is a complex process, and while these are not the only hormones involved in the system, they are two of the most vital. Research continues to explore the underlying mechanisms involved in homeostatic hunger, with advances being made all the time.

Recently, for example, POMC (Pro-opiomelanocortin) neurons in the hypothalamus have been identified as being key in limiting food intake, and recent studies have demonstrated that they change shape after eating due to hormonal fluctuations, amending the neurons' connections with neural circuitry.

Hedonic hunger

Homeostatic hunger is not the only system governing how much we eat. Is that was the case, then we would generally be at our ideal body weight, and eating would be akin to other essential life-supporting processes such as breathing or sleeping.

However, food and eating play a significant role in cultures around the world. We eat to celebrate, to enjoy ourselves, it’s a big part of socializing and is also significant in religious holidays and family events.

Humans are complex creatures, and in today’s modern society we are constantly subjected to unconscious motivators. Given that the food industry aims to make a profit from the sale of their goods, marketing to motivate us to consume can have the impact of initiating feelings of hunger.

Also, through stimulus-response learning, we associate eating with several scenarios (such as those listed above) that can trigger feelings of hunger even when our bodies do not require nutrition.

Much of this unnecessary feasting is driven by the pleasure associated with food, this is known as hedonic hunger. This system encompasses two mechanisms, gustatory mechanisms, and the reward system for feeding.

Gustatory mechanisms for regulating feeding are modulated by palatability cues such as taste and smell. Much research has shown that humans and animals will eat sweet and salty foods beyond what they need for adequate nutrition.

Scientists theorize that these are evolutionary behaviors that exist to increase our chances of survival because sweet and salty tastes are usually indicators of foods that are important for survival, such as those that are high in calories.

Today, these systems still impact us even though in the developed world we do not have to worry of we will find adequate food sources in the future. The brain still seeks out these sugary and salty foods that please us.

In addition, the brain has a reward system for feeding. Numerous studies have demonstrated that food rewards may share the neural pathways involved in the reward system in drug addiction. Evidence has shown that opioid receptors are key in both systems for delivering a sense of pleasure and reward in obtaining the substance, whether it is food or drugs.